SMASS: Difference between revisions

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{{TAGDEF|SMASS|-3 {{!}} -2 {{!}} -1 {{!}} [real] ≥ 0|-3}}
{{TAGDEF|SMASS|-3 {{!}} -2 {{!}} -1 {{!}} [real] ≥ 0|-3}}


Description: {{TAG|SMASS}} controls the velocities during an ab-initio molecular dynamics run.
Description: {{TAG|SMASS}} controls the velocities during an ab-initio molecular-dynamics run.
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* {{TAG|SMASS}}=-3
* {{TAG|SMASS}}=-3
:For {{TAG|SMASS}}=-3 a micro canonical ensemble is simulated (constant energy molecular dynamics). The calculated Hellmann-Feynman forces serve as an acceleration acting onto the ions. The total free energy (i.e. free electronic energy + Madelung energy of ions + kinetic energy of ions) is conserved.
:For {{TAG|SMASS}}=-3 a microcanonical ensemble ([[NVE ensemble]]) is simulated (constant energy molecular dynamics). The calculated Hellmann-Feynman forces serve as an acceleration acting onto the ions. The total free energy (i.e. free electronic energy + Madelung energy of ions + kinetic energy of ions) is conserved. {{NB|tip|To calculate an [[NVE ensemble]] we instead recommend to use {{TAGO|MDALGO|1}} and {{TAGO|ANDERSEN_PROB|0.0}}.|:}}


* {{TAG|SMASS}}=-2
* {{TAG|SMASS}}=-2
:For {{TAG|SMASS}}=-2 the initial velocities are kept constant. This allows to calculate the energy for a set of different linear dependent positions (for instance [[frozen phonons]], or [[dimers]] with varying bond-length).
:For {{TAG|SMASS}}=-2 the initial velocities are kept constant. This allows to calculate the energy for a set of different linear dependent positions (for instance frozen phonons, or dimers with varying bond lengths).
:'''Mind''': if {{TAG|SMASS}}=-2 the actual steps taken are {{TAG|POTIM}}×(velocities-read-from-the-{{FILE|POSCAR}}-file). To avoid ambiguities, set {{TAG|POTIM}}=1.
:'''Mind''': if {{TAG|SMASS}}=-2 the actual steps taken are {{TAG|POTIM}}×(velocities-read-from-the-{{FILE|POSCAR}}-file). To avoid ambiguities, set {{TAG|POTIM}}=1.


* {{TAG|SMASS}}=-1
* {{TAG|SMASS}}=-1
:In this case the velocities are scaled each {{TAG|NBLOCK}} step (starting at the first step i.e. MOD(NSTEP,{{TAG|NBLOCK}})=1) to the temperature: T={{TAG|TEBEG}}+({{TAG|TEEND}}-{{TAG|TEBEG}})×NSTEP/{{TAG|NSW}},
:In this case the velocities are scaled each {{TAG|NBLOCK}} step (starting at the first step i.e. MOD(NSTEP,{{TAG|NBLOCK}})=1) to the temperature: T={{TAG|TEBEG}}+({{TAG|TEEND}}-{{TAG|TEBEG}})×NSTEP/{{TAG|NSW}},
:where NSTEP is the current step (starting from 1). This allows a continuous increase or decrease of the kinetic energy. In the intermediate period a micro-canonical ensemble is simulated.
:where NSTEP is the current step (starting from 1). This allows a continuous increase or decrease of the kinetic energy. In the intermediate period, a micro-canonical ensemble is simulated.


* {{TAG|SMASS}}≥0
* {{TAG|SMASS}}≥0
:For {{TAG|SMASS}}≥0, a canonical ensemble is simulated using the algorithm of Nosé. The Nosé mass controls the frequency of the temperature oscillations during the simulation.{{cite|nose:jcp:1984}}{{cite|nose:ptp:1991}}{{cite|bylander:prb:1992}} For {{TAG|SMASS}}=0, a Nosé-mass corresponding to period of 40 time steps will be chosen. The Nosé-mass should be set such that the induced temperature fluctuation show approximately the same frequencies as the typical 'phonon'-frequencies for the specific system. For liquids something like 'phonon'-frequencies might be obtained from the spectrum of the velocity auto-correlation function. If the ionic frequencies differ by an order of magnitude from the frequencies of the induced temperature fluctuations, Nosé thermostat and ionic movement might decouple leading to a non canonical ensemble. The frequency of the approximate temperature fluctuations induced by the Nosé-thermostat is written to the {{FILE|OUTCAR}} file.
:For {{TAG|SMASS}}≥0, a canonical ensemble is simulated using the algorithm of Nosé. The Nosé mass controls the frequency of the temperature oscillations during the simulation.{{cite|nose:jcp:1984}}{{cite|nose:ptp:1991}}{{cite|bylander:prb:1992}} For {{TAG|SMASS}}=0, a Nosé-mass corresponding to period of 40 time steps will be chosen. The Nosé-mass should be set such that the induced temperature fluctuation show approximately the same frequencies as the typical 'phonon'-frequencies for the specific system. For liquids something like 'phonon'-frequencies might be obtained from the spectrum of the velocity auto-correlation function. If the ionic frequencies differ by an order of magnitude from the frequencies of the induced temperature fluctuations, Nosé thermostat and ionic movement might decouple leading to a non-canonical ensemble. The frequency of the approximate temperature fluctuations induced by the Nosé-thermostat is written to the {{FILE|OUTCAR}} file.


== Related Tags and Sections ==
== Related tags and articles ==
[[structure optimization]],
{{TAG|IBRION}},
{{TAG|IBRION}},
{{TAG|POTIM}},
{{TAG|POTIM}},
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[[Category:INCAR]][[Category:Molecular Dynamics]]
[[Category:INCAR tag]][[Category:Molecular dynamics]][[Category:Thermostats]]

Latest revision as of 11:50, 18 October 2024

SMASS = -3 | -2 | -1 | [real] ≥ 0
Default: SMASS = -3 

Description: SMASS controls the velocities during an ab-initio molecular-dynamics run.

For SMASS=-3 a microcanonical ensemble (NVE ensemble) is simulated (constant energy molecular dynamics). The calculated Hellmann-Feynman forces serve as an acceleration acting onto the ions. The total free energy (i.e. free electronic energy + Madelung energy of ions + kinetic energy of ions) is conserved.
Tip: To calculate an NVE ensemble we instead recommend to use MDALGO = 1 and ANDERSEN_PROB = 0.0.
For SMASS=-2 the initial velocities are kept constant. This allows to calculate the energy for a set of different linear dependent positions (for instance frozen phonons, or dimers with varying bond lengths).
Mind: if SMASS=-2 the actual steps taken are POTIM×(velocities-read-from-the-POSCAR-file). To avoid ambiguities, set POTIM=1.
In this case the velocities are scaled each NBLOCK step (starting at the first step i.e. MOD(NSTEP,NBLOCK)=1) to the temperature: T=TEBEG+(TEEND-TEBEG)×NSTEP/NSW,
where NSTEP is the current step (starting from 1). This allows a continuous increase or decrease of the kinetic energy. In the intermediate period, a micro-canonical ensemble is simulated.
For SMASS≥0, a canonical ensemble is simulated using the algorithm of Nosé. The Nosé mass controls the frequency of the temperature oscillations during the simulation.[1][2][3] For SMASS=0, a Nosé-mass corresponding to period of 40 time steps will be chosen. The Nosé-mass should be set such that the induced temperature fluctuation show approximately the same frequencies as the typical 'phonon'-frequencies for the specific system. For liquids something like 'phonon'-frequencies might be obtained from the spectrum of the velocity auto-correlation function. If the ionic frequencies differ by an order of magnitude from the frequencies of the induced temperature fluctuations, Nosé thermostat and ionic movement might decouple leading to a non-canonical ensemble. The frequency of the approximate temperature fluctuations induced by the Nosé-thermostat is written to the OUTCAR file.

Related tags and articles

structure optimization, IBRION, POTIM, NBLOCK, TEBEG, TEEND

Examples that use this tag

References

  1. ↑ S. Nosé, J. Chem. Phys. 81, 511 (1984).
  2. ↑ S. Nosé, Prog. Theor. Phys. Suppl. 103, 1 (1991).
  3. ↑ D. M. Bylander and L. Kleinman, Phys. Rev. B 46, 13756 (1992).


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